Butadiene is an important base chemical and is used, for example, to prepare synthetic rubbers (butadiene homopolymers, styrene-butadiene-rubber or nitrile rubber) or for preparing thermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers). Butadiene is also converted to sulfolane, chloroprene and 1,4-hexamethylenediamine (via 1,4-dichlorobutene and adiponitrile). Dimerization of butadiene also allows vinylcyclohexene to be generated, which can be dehydrogenated to form styrene.
Butadiene can be prepared from saturated hydrocarbons by refining process or by thermal cracking (steam cracking) processes, in which case naphtha is typically used as the raw material. In the course of refining or steam cracking of naphtha, a mixture of methane, ethane, ethene, acetylene, propane, propene, propyne, allene, butenes, butadiene, butynes, methylallene, C4 and higher hydrocarbons are obtained.
Typical processes to recover butadiene from mixed C4 streams include extractive distillation processes, which may incorporate use of selective solvents. Examples of extractive distillation processes are found, for example, in U.S. Pat. Nos. 7,393,992, 7,482,500, 7,226,527, 4,310,388, and 7,132,038, among others.
The butadiene recovery processes typically use 3- or 4-column extractive distillation systems to separate a mixed C4 stream into product fractions, including a lights/butane/butenes stream (Raffinate-1 product), a crude butadiene product, which may be sent to a conventional distillation system for further purification, and C3 acetylenes (propyne) and C4 acetylenes streams, which may be sent to a selective hydrogenation unit, for example.
The C3 and C4 acetylenes may be selectively hydrogenated downstream of the butadiene recovery process to form useful olefins and dienes. Formation of olefin oligomers (green oil) may result during such hydrogenation processes. Processing of the crude C4's at elevated temperatures through extractive and conventional distillation may also result in formation of dimers and oligomers, such as vinyl cyclohexane. In other processes for the recovery of butadiene, the totality of a crude C4 stream may be passed through a selective hydrogenation reactor to selectively hydrogenate acetylenes upstream of extractive distillation. Following such hydrogenation systems, it is typical to separate the oligomeric “green oil” byproducts from the desired hydrocarbon products using a separator commonly referred to as a green oil column.
In addition to the acetylene streams, 1,2-butadiene and C5+ components are also recovered from the butadiene separation processes. Other waste streams may also be produced during the various butadiene recovery processes. For example, butadiene vaporization may require the removal of non-vaporized components from a vaporizer drum.